Field of the Disclosure
Embodiments of the present invention relate to an organic light emitting display device. More particularly, the invention relates to an organic light emitting display device that compensates for variations of a driving transistor in each pixel of the display device.
Discussion of the Related Art
Due to recent developments in multimedia, there is an increasing demand for flat panel displays. In order to satisfy the increasing demand, various flat panel displays such as liquid crystal displays, plasma display panels, field emission displays, and organic light emitting displays are practically used. Among the various flat panel displays, the organic light emitting display device is attractive as a next-generation flat panel display because of its rapid response speed and low power consumption. In addition, the organic light emitting display device self-emits light and does not cause a problem related with narrow viewing angles.
FIG. 1 is a circuit diagram illustrating a pixel structure of an organic light emitting display device according to the related art.
Referring to FIG. 1, a pixel (P) of the organic light emitting display device according to the related art may include a switching transistor (Tsw), a driving transistor (Tdr), a capacitor (Cst), and an organic light emitting diode (OLED).
The switching transistor (Tsw) is switched by a scan pulse (SP) supplied via a scan line (SL), the switching transistor (Tsw) supplies a data voltage (Vdata) supplied via a data line (DL) to the driving transistor (Tdr). The driving transistor (Tdr) is switched by the data voltage (Vdata) supplied from the switching transistor (Tsw), to control a data current (Ioled) flowing from a driving power source (EVdd) to the organic light emitting diode (OLED). The capacitor (Cst) is connected between gate and source terminals of the driving transistor (Tdr), wherein the capacitor (Cst) stores a voltage corresponding to the data voltage (Vdata) supplied to the gate terminal of the driving transistor (Tdr), and turns on the driving transistor (Tdr) with the stored voltage. The organic light emitting diode (OLED) is electrically connected between the source terminal of the driving transistor (Tdr) and a cathode line (EVss), wherein the organic light emitting diode (OLED) emits light from the data current (Ioled) supplied from the driving transistor (Tdr). In each pixel (P) of the organic light emitting display device according to the related art, a level of the data current (Ioled) flowing from the driving power source (EVdd) to the organic light emitting diode (OLED) is controlled by switching the driving transistor (Tdr) according to the data voltage (Vdata) so that the organic light emitting diode (OLED) emits light to display a predetermined image.
In the case of the organic light emitting display device according to the related art, the characteristics of threshold voltage (Vth)/mobility in the driving transistor (Tdr) may different at each pixel position of an organic light emitting display panel due to non-uniformity of the thin-film transistors caused by manufacturing process variation. Accordingly, even though the same data voltage (Vdata) is applied to the driving transistor (Tdr) for each pixel in the organic light emitting display device according to the related art, it is difficult to realize uniformity in picture quality due to deviations of the current flowing in the organic light emitting diode (OLED).
In order to overcome these problems, Korean Patent Publication Number 10-2012-0076215 (hereinafter, referred to as ‘prior-art document’) discloses an organic light emitting display device and an external compensation method in which a sensor transistor is additionally provided in each pixel, a threshold voltage of a driving transistor is sensed through the use of a reference line connected to the sensor transistor by switching the sensor transistor and switching transistor, and variations of the threshold voltage of the driving transistor are compensated.
In the organic light emitting display device of the prior-art document, the change of threshold voltage in the driving transistor is sensed based on a value sensed in an analog-to-digital converter by driving the driving transistor for each pixel in a source follower mode, and then the threshold voltage of the driving transistor is compensated.
However, in the case of the above sensing method of the prior-art document, a sensing voltage is saturated after a sufficient time period due to an influence of capacitance. Thus, in order to accurately compensate for the threshold voltage of the driving transistor, the voltage saturation should be delayed until the driving transistor is turned-off. However, this causes sensing speed to be slower due to the increase of sensing time.